A hollow extrusion die set commonly consists of a hollow female die member and a solid male die member which are mated together to form spaced-apart extrusion bearing surfaces shaped to determine the sectional shape and dimension of extrusion-formed products.
It is known that solid extrusion die sets are efficiently and economically machined by an existing numerical controlled WC-EDM machine. The art has recognized heretofore that hollow extrusion die sets, however, are not effectively machinable by these machines. Thus, a hollow extrusion die set has hitherto been prepared only by the extremely complex and costly procedure which includes the step of pre-machining the male and female die blanks separately by electrochemical machining (ECM) or electrical discharge machining (EDM) of the cavity-sinking type, followed by the steps of assembling the separately machined die blanks, heat-treatment, grind-finishing the bearing surface again by EDM and finally hand-finishing to a desired precision and final finish. Not only does the conventional practice of making a hollow extrusion die set entail numerous steps which are complicated but it is quite problematical in that it requires the provisional assembling of the rough-machined die blanks before heat-treatment and necessitates one or more electrodes for EDM operations to provide the intricate bearing surfaces. Furthermore, in a cavity-sinking type EDM process, a completely straight cut is not obtainable and a taper is unavoidably left on the bearing surfaces. This entails repeated inspection and redressing steps and makes it difficult to provide separate sets of dies which can be interchanged.